Foundries for the metal-casting industry generate by-products such as used foundry sand. In fact, foundries successfully recycle and reuse sand many a times, and the remaining sand, which is foundry sand, is removed from the foundry. This sand is uniformly sized, high-quality silica sand combined with a binder and used to form moulds for ferrous and non-ferrous castings.

The properties of used foundry-sand properties vary owing to the type of equipment used for foundry processing, types of additives, number of times the sand is reused, and the type and amount of binder. Within the concrete industry, the most successful examples have been using coal fly-ash to make high-quality, durable concrete and recycling old, demolished concrete as aggregate for new concrete.

The Indian foundry industry: India ranks second in the world based on the number of foundry units that exist (4,550 units) and fourth in terms of total production (7.8 million tonne), according to the 42nd Census of World Casting Production, 2007. Apart from the registered 4,550 units, there are several unregistered units, which according to sources range from 1,500 to 5,000 units. Today, India ranks 10th among casting exporters. Foundries produce a wide variety of castings such as manhole covers, pipe and pipe fittings, sanitary items, tube-well bodies, metric weights, automobile components, railway parts, electric motors, fan bodies, etc; 90 per cent of the castings produced are from the SSI sector. Most of these units are situated in clusters, with cluster size ranging from 30 to 500 units.

The applications: Foundry sand is a by-product of ferrous and non-ferrous metal casting industries, where sand has been used for centuries as a moulding material because of its thermal conductivity. Foundry sand consists primarily of high-quality silica sand with uniform physical characteristics, coated with a thin film of burnt carbon, residue and dust. It can be used in concrete to improve its strength and other durability factors. It can be used as a partial replacement of cement or as a partial replacement of fine aggregates or total replacement of fine aggregate and as supplementary addition to achieve different properties of concrete.

Foundry sand can be used in embankments, barrier layers construction, flowable fills, roads construction, as soil reinforcement, in hot mix asphalt and Portland cement concrete. Further, engineering applications include Portland cement manufacturing, mortars, agriculture or soil amendments, verification of hazardous materials, smelting, rockwool and fibreglass manufacturing and landfill covers or hydraulic barriers. However, limitations include the ability to meet the quantity requirements of precast manufacturers and reduction in workability of concrete.

That said, it can be suitable for a variety of reuses such as:

Structural fill: Foundry sand can be used as support for structures such as roadways, parking lots, buildings, and pieces of equipment. ´Encapsulated´ structural fill may involve the use of a liner, cap or cover, generally made of clay, which prevents water from percolating through the foundry sand and minimises the potential for leaching.

- Flowable fill: Flowable fill is a liquid-like material that self-compacts and is used as a substitute for conventional soil backfill. The product is easily transported and can be readily re-excavated. The typical mixture contains sand, fly-ash, Portland cement and water. Foundry sand can readily be substituted for virgin sand in flowable fill mixtures.
- Cement and concrete: Sand is a component of Portland cement and concrete. Portland cement requires sand with a silica content of at least 80 per cent, which most foundry sands meet. It also requires certain minerals such as iron and aluminium oxides, which are found in many foundry sands. Cement and additional sand or gravel are components of concrete, allowing further reuse of foundry sand.

Soil manufacturing and amendment: Commercial soil blending operations can use foundry sand to produce horticultural soils, topsoil, potting soil and turf mixes. These soil products are typically mixtures of sand or gravel with peat, fertilisers or top soil. Foundry sand can also improve the performance of agricultural soils and be used as a composting ingredient.

Landfill uses: Foundry sand can be used as a cover for the working face of an active landfill, for road construction within the active cell, or as a substitute for virgin aggregate in the construction of drainage layers for landfill leachate collection systems.

Pipe bedding and backfill: Foundry sand can serve as backfill for trenches created by the installation of storm and sanitary sewer lines.

Types and physical characteristics: Two general types of binder systems are used in metal casting:

Green sand moulds: These are used to produce about 90 per cent of casting volume in the US. Green sand is composed of naturally occurring materials that are blended together; high-quality silica sand (85-95 per cent), bentonite clay (4-10 per cent) as a binder, a carbonaceous additive (2-10 per cent) to improve the casting surface finish; and water (2-5 per cent). Green sand is the most commonly used recycled foundry sand for beneficial reuse. It is black in colour owing to carbon content, has a clay content that results in percentage of material that passes a 200 sieve and adheres owing to clay and water.

Chemically bonded sands: These are used in both core making, where high strengths are necessary to withstand the heat of molten metal, and mould making. Most chemical binder systems consist of an organic binder activated by a catalyst although some systems use inorganic binders. Chemically bonded sands are generally lighter in colour and texture than clay bonded sands.

Foundry sand is typically sub angular to round in shape. After being used in the foundry process, a significant number of sand agglomerations form. When these are broken down, the shape of individual sand grains is apparent. Green sands are typically black, or gray, not green; chemically bonded sand is typically a medium tan or off-white colour.

In conclusion: Concerning the compressive strength and flexural strength test of concrete (M20 Grade) for rigid pavement, here are some observations: The maximum compressive strength, flexural strength is achieved at 50 per cent replacement of natural fine aggregate with used foundry sand, which is 40.89 N per sq mm and 8.45 N per sq mm respectively; California Bearing Ratio (CBR) value 4 per cent, Dry Lean Concrete (DLC) = 100 mm thick, so cost of rigid pavement decreases from Rs 757.04 to Rs 632.17. Hence, the use of foundry sand in concrete can save the ferrous and non-ferrous metal industries disposal, cost and produce a ´greener´ concrete for construction. Further, environmental effects from wastes and disposal problems of waste can be reduced. Indeed, used foundry sand can be an innovative construction material if judicious decisions are taken by engineers.

CASE STUDY
A mix M20 grade was designed as per Indian Standard method (IS: 10262-1982) and the same was used to prepare the test samples. The design mix proportion is done in Table 1.

DESIGN OF A ROAD PAVEMENT (IRC: 58-2002)
A cement concrete pavement is to be designed for a two-lane two-way National Highway in Gujarat. The total two-way traffic is 3,000 commercial vehicles per day (cvpd) at the end of the construction period.